Fix #8554: silently ignore missing annotations like javac

compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala

@@ -534,6 +534,15 @@ class ClassfileParser(
    */
   def parseAnnotation(attrNameIndex: Char, skip: Boolean = false)(implicit ctx: Context): Option[Annotation] = try {
     val attrType = pool.getType(attrNameIndex)
+    attrType match {
+      case tp: TypeRef =>
+        // Silently ignore missing annotation classes like javac
+        if tp.denot.infoOrCompleter.isInstanceOf[StubInfo] then
+          if (ctx.debug)
+            ctx.warning(i"Error while parsing annotations in ${in.file}: annotation class $tp not present on classpath")
+          return None
+      case _ =>
+    }
     val nargs = in.nextChar
     val argbuf = new ListBuffer[untpd.Tree]
     var hasError = false

compiler/test/dotty/tools/AnnotationsTests.scala

@@ -37,3 +37,20 @@ class AnnotationsTest:
         }
       }
     }
+
+  @Test def surviveMissingAnnot: Unit =
+    withJavaCompiled(
+      VirtualJavaSource("Annot.java",
+        "public @interface Annot {}"),
+      VirtualJavaSource("A.java",
+        "@Annot() public class A {}")) { javaOutputDir =>
+      Files.delete(javaOutputDir.resolve("Annot.class"))
+      inCompilerContext(javaOutputDir.toString + File.pathSeparator + TestConfiguration.basicClasspath) {
+        val cls = ctx.requiredClass("A")
+        val annots = cls.annotations.map(_.tree)
+        assert(annots == Nil,
+          s"class A should have no visible annotations since Annot is not on the classpath, but found: $annots")
+        assert(!ctx.reporter.hasErrors && !ctx.reporter.hasWarnings,
+          s"A missing annotation while parsing a Java class should be silently ignored but: ${ctx.reporter.summary}")
+      }
+    }